1. Field of the Invention
This invention relates to laser shock peening and, more particularly, to methods of simultaneously laser shock peening opposite sides of an article using offset low energy laser beams.
2. Background Art
Laser shock peening or laser shock processing, as it is also referred to, is a process for producing a region of deep compressive residual stresses imparted by laser shock peening a surface area of an article. Laser shock peening typically uses one or more radiation pulses from high energy, about 50 joules or more, pulsed laser beams to produce an intense shockwave at the surface of an article similar to methods disclosed in U.S. Pat. No. 3,850,698 entitled xe2x80x9cAltering Material Propertiesxe2x80x9d; U.S. Pat. No. 4,401,477 entitled xe2x80x9cLaser Shock Processingxe2x80x9d; and U.S. Pat. No. 5,131,957 entitled xe2x80x9cMaterial Propertiesxe2x80x9d. Laser shock peening, as understood in the art and as used herein, means utilizing a pulsed laser beam from a laser beam source to produce a strong localized compressive force on a portion of a surface by producing an explosive force at the impingement point of the laser beam by an instantaneous ablation or vaporization of a thin layer of that surface or of a coating (such as tape or paint) on that surface which forms a plasma.
Laser shock peening is being developed for many applications in the gas turbine engine field, some of which are disclosed in the following U.S. Pat. No. 5,756,965 entitled xe2x80x9cOn The Fly Laser Shock Peeningxe2x80x9d; U.S. Pat. No. 5,591,009 entitled xe2x80x9cLaser shock peened gas turbine engine fan blade edgesxe2x80x9d; U.S. Pat. No. 5,531,570 entitled xe2x80x9cDistortion control for laser shock peened gas turbine engine compressor blade edgesxe2x80x9d; U.S. Pat. No. 5,492,447 entitled xe2x80x9cLaser shock peened rotor components for turbomachineryxe2x80x9d; U.S. Pat. No. 5,674,329 entitled xe2x80x9cAdhesive tape covered laser shock peeningxe2x80x9d; and U.S. Pat. No. 5,674,328 entitled xe2x80x9cDry tape covered laser shock peeningxe2x80x9d, all of which are assigned to the present Assignee.
Laser peening has been utilized to create a compressively stressed protective layer at the outer surface of an article which is known to considerably increase the resistance of the article to fatigue failure as disclosed in U.S. Pat. No. 4,937,421 entitled xe2x80x9cLaser Peening System and Methodxe2x80x9d. These methods typically employ a curtain of water flowed over the article or some other method to provide a plasma confining medium. This medium enables the plasma to rapidly achieve shockwave pressures that produce the plastic deformation and associated residual stress patterns that constitute the LSP effect. The curtain of water provides a confining medium, to confine and redirect the process generated shockwaves into the bulk of the material of a component being LSP""D, to create the beneficial compressive residual stresses.
The pressure pulse from the rapidly expanding plasma imparts a traveling shockwave into the component. This compressive shockwave caused by the laser pulse results in deep plastic compressive strains in the component. These plastic strains produce residual stresses consistent with the dynamic modules of the material. Dual sided simultaneous laser shock peening includes simultaneously striking both sides of an article by two laser beams in order to increase the compressive residual stress in the material. The laser beams are typically balanced in order to minimize material distortion. The initial compressive waves pass through the material from each of the sides and are reflected back from the interface of the two initial compressive waves. The reflected waves turn into a tension wave. The combined tensile stress of the reflected waves, when the reflected tension waves from the both sides meet at mid-point in the same axial direction, can be greater than the strength that the material can handle and a crack can be initiated at the mid-plane where the two shockwaves meet.
Another characteristic of LSP that limits its engineering effectiveness is the formation of deleterious release waves that create tensile strains. The released waves may form spontaneously following the compressive front or may result from reflection at a surface with impedance mismatch such as at the outer surface of a component being laser shock peened. When multiple release waves are simultaneously propagating in a component, they may add in a manner termed superposition. This superposition of tensile waves may reduce the effectiveness of the beneficial compressive strains or may even cause tensile fracture within the component. This superposition of the two spatially concentric waves thus reduces the beneficial effects which may be measured by HCF testing.
Thus, it is highly desirable to have a process for and to produce an article that is simultaneously laser shock peened on two opposite sides and eliminate the mid-plane cracks by lowering the combined tensile stress of the reflected waves just below the maximum or allowable tensile stress of the material. It is also highly desirable to be able to eliminate or reduce loss of HCF benefits or effectiveness of the beneficial compressive strains from laser shock peening caused by the superposition of tensile waves.
Manufacturing costs of the laser shock peening process is a great area of concern because startup and operation costs can be very expensive. The use of low energy laser beams of this order of magnitude is disclosed in U.S. Pat. No. 5,932,120, entitled xe2x80x9cLaser Shock Peening Using Low Energy Laserxe2x80x9d, which issued Aug. 3, 1999 and is assigned to the present assignee of this patent and is incorporated herein by reference. Manufacturers are constantly seeking methods to reduce the time, cost, and complexity of such processes and it is also to this end that the present invention is directed.
A method for laser shock peening an article includes aiming and then simultaneously firing first and second low energy laser beams with sufficient energy to vaporize material on longitudinally spaced apart first and second surface portions of the article to form first and second regions having deep compressive residual stresses extending into the article from the first and second laser shock peened surface portions, respectfully. The low energy laser beams have low energy levels on the order of 3-10 joules or even perhaps 1-10 joules to allow smaller less expensive lasers to be used as disclosed in U.S. Pat. No. 5,932,120, entitled xe2x80x9cLaser Shock Peening Using Low Energy Laserxe2x80x9d. The present method uses low energy laser beams having an output in a range of about 1-10 joules. An energy level range of about 3-7 joules has been found particularly effective as has an energy level of about 3 joules. The low energy beams are focused to produce small diameter laser spots having a diameter in a range of about 1 mm (0.040 in.) to 2 mm (0.080 in.). In one embodiment, the first and second laser beams are aimed such that first and second centerlines of the first and second laser beams impinge the first and second surface portions at first and second laser beam centerpoints through which pass parallel first and second axes that are substantially normal to the first and second surface portions at the first and second laser beam centerpoints, respectfully, and such that the first and second axes that are offset. In a first more particular embodiment of the present invention, the first and second laser beams are aimed such that the first and second centerlines intersect and are angled with respect to each other. In a second more particular embodiment of the present invention, the first and second laser beams and the first and second centerlines are parallel and offset with respect to each other.
Another more particular embodiment of the present invention, the laser beams are aimed and fired in a manner to produce first and second patterns on the first and second surface portions of the article having overlapping adjacent rows of overlapping adjacent one of the first and second spots, respectively. The patterns are formed by continuously moving the article, while holding stationary and continuously firing the laser beams with repeatable pulses with relatively constant periods between the pulses, wherein the surface portions are laser shock peened using sets of sequences, and wherein each sequence includes continuously firing the laser beams on the surfaces such that on each of the surface portions adjacent ones of the laser shock peened spots are hit in different ones of the sequences in the sets. A more particular embodiment includes coating the surface portions with an ablative coating before and in between the sequences in the set.
In one more embodiment of the present invention, the article is a gas turbine engine airfoil and the first and second surface portions are on pressure and suction sides, respectively, of the airfoil along a leading edge of the airfoil.
The present invention includes a laser shock peened article having laser shock peened first and second surface portions with first and second regions having deep compressive residual stresses extending into the article from the first and second laser shock peened surface portions, respectfully, wherein the first and second surface portions comprise couples of simultaneously laser shock peened first and second spots from laser shock peening, and each couple of the simultaneously laser shock peened first and second spots are longitudinally spaced apart and transversely offset from each other. In one embodiment of the present invention, the couple of the simultaneously laser shock peened first and second spots are substantially parallel. In one more particular embodiment of the present invention, the first and second surface portions of the article include first and second patterns of overlapping adjacent rows of overlapping adjacent ones of the first and second spots, respectively.
The present invention has many advantages including lowering the cost, time, man power and complexity of performing laser shock peening by allowing crack free dual sided simultaneous laser shock peening. The present invention provides a dual sided simultaneous laser shock peening method which is able to eliminate the mid-plane cracks by lowering the combined tensile stress of the reflected waves below the maximum or allowable tensile stress of the material. The invention provides a simultaneously dual sided laser shock peened article without the mid-plane cracks. The invention is also advantageous because it can be used to eliminate or reduce loss of HCF benefits or effectiveness of the beneficial compressive strains from laser shock peening caused by the superposition of tensile waves. The invention has been found useful to provide a positive effect on HCF capability of laser shock peened articles and in particular laser shock peened leading edges of airfoils gas turbine engine blades and vanes.